Bactericidal properties of eight chemicals were tested in-vitro against onion soft rot bacteria (Burkholderia cepacia). Among the chemicals, acetic acid, boric acid and bleaching powder showed bactericidal activity against onion soft rot bacteria, B. cepacia O-15. These three chemicals were tested to treat onion bulbs against soft rot disease in storage. For treatment, fresh onion bulbs were dipped in 0.00, 0.02, 0.05 and 1.00% solutions of acetic acid, boric acid, and bleaching powder for 30 min. The treated bulbs were inoculated by spraying B. cepacia suspension and stored for 2-22 weeks. During the storage period, the chemicals caused 0.0-18.2% reduction in soft rot incidence and 0.00-18.0% reduction in weight loss of onion bulbs over to control. The lowest incidence of soft rot as well as weight loss was achieved with bleaching powder followed by boric acid and acetic acid. Results of both in vitro test and onion treatment experiment indicate that bleaching powder, boric acid and acetic acid may be used to control soft rot of onion bulbs during storage for a period of 2-22 weeks.

Bacterial soft rot (Burkholderia cepacia) is a common post harvest disease of onion and many other vegetables throughout the world. The disease causes severe loss of onion bulb in storage (Bdliya and Haruna 2007). Normally, chemical bactericides are not recommended for the control of soft rot disease because of high risk of health hazards (Agrios 1997). However, many scientists tested various bactericides including chemicals and microbial pesticides to control the soft rot bacteria (Chen and Lin 2000, Abd-El-Khair 2004, Wright et al. 2005). Researchers identified some chemicals with antimicrobial activity, which increase resistance in potato and onion against soft rot disease (Hammerschmidt and Smith 1997). Benzothiodiazole (BTH) has been idenfied as a systemic resistance inducer in many plants and effective against various plant pathogens (Gorlach et al. 1996, Bokshi et al. 2003). Increased resistance in potato tubers against E. carotovora subsp. carotovora was observed when tubers were dipped in acetyl salicylic acid (Abd-El-Sayed et al. 1996, Bokshi et al. 2003). Salt treatments also can inhibit plant pathogens or suppress their toxin production (Olivier et al. 1998). Salts including calcium propionate and calcium chloride reduced tissue maceration of potato tubers caused by E. carotovora (McGuire and Kelman 1986, Biggs et al. 1997, Droby et al. 1997).

Suppression of bacterial soft rot in potato tubers by application of an antibiotic ‘kasugamycin’ was investigated by Bartz (1999). Reports on chemical control of soft rot bacteria are not available in Bangladesh. Search for selection of chemicals without health hazard to human is necessary to control soft rot of onion.

Considering the above facts the present investigation was conducted to test some chemical substances for their effectiveness to control soft rot causing bacterial pathogens of onion.

The YPDA was prepared following a standard method as described by Tuite (1969). After cooking the medium was amended with appropriate quantity of each chemical to have desired levels of concentrations. Each chemical was added to YPDA, mixed thoroughly and autoclaved for 20 min at 121C under 1.1 kg/cm2 pressures. YPDA without any chemical amendment served as control. After sterilization, the medium was poured into 90 mm glass Petri dishes at 20 ml/plate and allowed to solidify.

To prepare the inocula, B. cepacia O-15 was grown on YPDA at 28C for 24 hr. Bacterial cells were collected from the culture and suspended in sterilized distilled water to a concentration of ca.108 cfu/ml. After solidification, YPDA in the plates was inoculated with bacterial suspension and incubated at 30C in an incubator. The plates were arranged in an incubator following completely randomized design with three replications. Three additional plates having YPDA without any chemical were maintained as control. Growth of the test bacteria in the plates was observed up to 14 days of inoculation and antibacterial activity of the chemicals was determined based on initiation of colony growth.

Efficacy of the chemicals to control soft rot

Based on the results of the in vitro test another experiment was conducted to evaluate the efficacy of acetic acid, boric acid and bleaching powder to control soft rot disease of onion in storage. Apparently healthy bulbs of onion variety ‘Taherpuri’ were selected and treated with the chemicals at 0.00, 0.20, 0.05 and 1.00% concen-trations. For each concentration of every chemical, 700 g of fresh onion bulbs were treated by dipping in solution of each chemical separately for 30 min and then air dried.

Fresh cultures of B. cepacia O-15 grown on YPDA were suspended in sterilized distilled water to prepare inocula at a concentration of ca.108 cfu/m. Onion bulb treated with chemicals were inoculated with the inoculum suspension of the bacteria using an atomizers and air dried again. The onion bulbs were packed in net bags and stored at room temperature for 22 weeks. For control treatment onion bulbs were treated with plain water, air dried and inoculated with the pathogen. The bulbs were checked for soft rot incidence on 2, 6, 10, 14, 18 and 22 weeks after inoculation. Data on soft rot infection and loss of weight due to soft rot in storage were recorded and expressed as percentage using the following formula described by Abd-El-Khair and Karima (2007):

RESULTS AND DISCUSSION

In vitro evaluation of eight chemicals against soft rot bacteria

Among eight chemicals tested in-vitro only acetic acid, boric acid and bleaching powder showed antibacterial activity against the soft rot bacteria. In-vitro growth of B. cepacia O-15 was inhibited by boric acid at all three concentrations (0.02, 0.05, 1.00%), acetic acid at two higher concentrations (0.05, 0.10%) and by bleaching powder only at the highest concentration of 1.00% (Table 1).

Efficacy of chemicals to control soft rot

At 2, 6, 10, 14, 18 and 22 weeks of storage, the soft rot incidence was 14.8, 42.9, 75.00, 82.10, 100.00 and 100.00% and loss in weight of onion bulb was 17.10, 43.5, 883.9, 100 and 100%, respectively. At different weeks of storage, application of three chemicals caused 0.0-18.2% reduction in soft rot incidence (Fig. 1) and 0.00-18.0% reduction in weight loss of onion bulbs (Fig. 2) over control. The lowest incidence of soft rot as well as weight was achieved with bleaching powder followed by boric acid and acetic acid (Fig. 1 and 2).

Results of the in-vitro test reveal that the most effective chemical to inhibit in-vitro growth of soft rot causing bacteria (B. cepacia O-15) of onion was bleaching powder followed by boric acid and acetic acid. Other investigators also found that organic acids can inhibit growth of B. cepacia causing soft rot of potato (Bokshi et al. 2003). Based on findings of the present in vitro test acetic acid, boric acid and bleaching powder were selected for treatment of onion to control soft rot disease during storage. Findings of the experiment reveal that prestorage treatment of onion bulbs with bleaching powder, boric acid and acetic acid is effective to reduce soft rot incidence and weight loss of onion bulbs. Many researchers also reported similar results. Hajhamed et al.(2007) found that potassium sulfate, ammonium phosphate and calcium chloride as salt compounds significantly decreased severity of bacterial soft rot disease of potato. Saleh and Huang (1997) reported that benzoic acid and sodium benzoate at 1, 5 and 10 mM inhibited soft rot bacterial growth and were effective in controlling the disease in both tomato fruits and potato tubers. Salts including calcium propionate and calcium chloride reduced tissue maceration of potato tubers due to attack of E. carotovora (Biggs et al. 1997, Droby et al. 1997, Olivier et al. 1998). Findings of the two experiments indicate that acetic acid, boric acid and bleaching powder may be recommended to control soft rot of onion caused by B. cepacia.

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The Bangladesh Journal of Plant Pathology (BJPP) is peer-reviewed international journal under the sponsorship of the Bangladesh Phytopathological Society (BPS). Sometimes the BPS receives financial assistance from the Ministry of Science and Technology, Peoples’ Republic of Bangladesh for its publication.